This invention relates to tracking of rotor blades, as for example helicopter rotor blades, and more particularly concern methods and apparatus to obtain accurate tracking despite the existence of conditions transiently altering rotor angular velocity.
The enablement of accurate dynamic smoothing of a rotor, such as a helicopter rotor, depends upon reduction or elimination of rotor blade "out-of-track". The term "track" refers to the requirement that the blade tips (or other corresponding blade portions) pass through the same plane (or fly at the same height) at a given or selected azimuth angle. If one of the two or more blades is above or below the others, it causes a vertical one-per-rev vibration of the "airframe" when the helicopter is in flight. This vibration generally increases as the speed of the helicopter is increased (rotor RPM is generally constant).
The above is true if the blades are "perfect", but sometimes the vertical vibration is less (the ride is better) if the blades are slightly out of track. Since minimum vibration is the objective, one can measure the vertical vibration with an accelerometer and determine not only vibration amplitude, but its phase relative to rotor azimuth position, as sensed by a magnetic pickup or photo cell on the hub (swash plates) of the rotor. A "tracking chart" or programmed calculator can then be employed to enable the user to adjust the track, using pitch link or trim tab, to optimize the ride. See in this regard U.S. Pat. Nos. 4,053,123 and 4,112,774.
A "Strobex" type stroboscope typically is used:
(1) to track rotor blades; PA0 (2) as a phase meter, when triggered from the vibration signal as measured by an accelerometer, which facilitates determination of weight addition locations; PA0 (3) as a speed (RPM) measuring strobe; and PA0 (4) as a strobe to view rotating of vibrating objects in slow motion. PA0 (a) repeatedly displaying corresponding portions of the rotating blades to have selected momentary positions in space, and PA0 (b) adjusting the repetition rate of such displaying as a function of transient force induced variation in drive angular frequency caused by the transient forces, thereby substantially to maintain the selected positions in space, azimuthally. PA0 (a) display means for repeatedly displaying corresponding portions of the rotating blades to have selected positions in space, and PA0 (b) circuitry operatively connected with the display means for adjusting the repetition rate of the displaying as a function of transient force induced variations in drive angular frequency caused by the transient forces, thereby substantially to maintain such selected positions in space, azimuthally.
In its tracking mode, it is often slaved to rotor blade rate (rotor RPM times number of blades) via a magnetic pickup secured to the helicopter fixed swash plate, and one interrupter per blade on the rotating swash plate. With each interrupter passage, an electrical pulse from the magnetic pickup causes the Strobex to flash. The magnetic pickup and interrupter are sometimes placed on other non-rotating and rotating members, and they can be replaced with other transducers such as photocell and reflector, etc.
With retro-reflective targets attached to the blade tips, the collimated light from the parabolic reflector and point-source is reflected back to the source (retro-reflected). If the Strobex (light source) is held nearly in front of the viewer's eyes, he sees the tip targets brightly. Because of the short duration of the flash (approximately 10 micro sec) the targets appear stopped, and always at the same azimuth position because of the triggering from the magnetic pickup. Since there is one interrupter per blade, all blades appear superimposed at certain azimuth positions, i.e. with two blades the "pattern" can be seen at 12:00 and 6:00 o'clock, three blades will be seen at 12:00, 4:00 and 8:00, four blades at 12:00, 3:00, 6:00 and 9:00 etc.
With helicopters having 4, 5, 6, or even 7 blades, it is desirable not to have the tip targets superimposed as previously described, but to be able to "spread" the pattern for easy viewing and identification of the targets. In some cases, the interrupters are slightly offset to achieve this effect, but such interrupter placement is very sensitive because they are typically placed at a very short radius, relative to the axis of rotor rotation, and the "leverage" at the large radius of the blade tips (typically 20:1) magnifies any errors in interrupter spacing. Also some swash plate mechanisms do not lend themselves to the mounting of one interrupter per blade.
One approach to this problem was the Chadwick-Helmuth Model 135M-9 Strobex and subsequently the Model 135M-11. They feature a "locking oscillator" set to flash the strobe at the one-per-blade rate, but "re-started" once per revolution by a single interrupter on the rotating member, passing by the fixed magnetic pickup.
It can be seen that a free-running oscillator, set exactly to the blade rate, (no magnetic pickup signal) will show all the blades superimposed at some fixed (but unknown) azimuth position. If the flash rate is slightly changed, all the blades will remain superimposed, but will rotate around the rotor disc, left or right. This is not a useful display because the targets are hard to find and follow, and because they cannot be spread.
If one adjusts the oscillator to flash once-per-blade to be re-started once per revolution, each time the single interrupter passes the magnetic pickup, it can be seen that the targets will always appear at the same azimuth position, and that they will be "stacked" if the flash rate exactly matches the one-per-blade rate. If one reduces the flash rate of the Strobex (increases the interval between flashes), the "master" blade will appear at its fixed azimuth position because the interrupter causes that flash--and starts the oscillator. The following blade will travel further than 1/4th of a revolution before the next flash because the flash interval is longer, and that blade's target will appear to the left of the first (the blades travel from right to left, except on French and Russian helicopters). The third blade will travel still farther and its target will appear to the left of the second, and so on. Thus, one can spread the blades or "stack" them, as desired by controlling the decrease of flash rate.
If the Strobex is caused to flash faster than blade rate, the pattern "goes to pieces" because the oscillator causes the light to flash before the locking pulse occurs. The pattern disappears off to the right and is useless.
While such a locking oscillator is advantageous, there is a problem. If rotor RPM transiently changes, as it often does with changes in power as in turns, climbs, descents, wind gusts, etc., the blade pattern will change its spread, or disappear, the same as if the flash rate is changed by the oscillator. Therefore, the user must continuously and "delicately" adjust the flash rate in order to keep the pattern as desired. This requires constant attention and skill, and it is very difficult, and distracting to try to "keep the pattern" in turbulence and during maneuvers (when it is especially desirable to see the targets).